Mixers are an important circuit for radio frequency (RF) transmitters and receivers. The function of a mixer is to perform a frequency translation. While the present invention is generally related to and applicable to a variety of RF transmitters and receivers, one of applications is so-called direct-conversion receiver, which is also known as a homodyne receiver. In a direct-conversion receiver, a mixer is used to convert an RF signal into a baseband signal by mixing the RF signal with a clock signal, which is usually referred to as a local oscillator (LO) signal. The frequency of the LO signal is nominally the same as the center frequency of the RF signal, and consequently the resultant baseband signal is centered at DC. In a twist known as a low-IF (intermediate frequency) receiver, which is also relevant to the present invention, the frequency of the LO signal is slightly different from the center frequency of the RF signal, and consequently the resultant baseband signal is centered at a low intermediate frequency. By way of example but not limitation, a down-conversion mixer suitable for direct-conversion receivers or low-IF receivers is used to describe the related art and to demonstrate how the present invention can be applied to improve the performance over the related art.
There are numerous topologies to construct a mixer circuit. A topology of particular relevance to the present invention is based on commutating a differential signal in a manner controlled by a differential LO signal. FIG. 1 depicts a commutation network 100. On one side, the commutation network 100 is coupled to an input differential signal comprised of VI+ and VI−; on the other side, it is coupled to an output differential signal comprised of VO+ and VO−. The commutation network 100 comprises four switches, embodied by four NMOS (N-type metal-oxide semiconductor) field-effect transistors M1, M2, M3, and M4, respectively. The commutation network 100 receives a differential LO signal comprised of VLO+ and VLO−. Each switch is controlled by either VLO+ or VLO−. Each switch is closed when its controlling signal is high, and open otherwise. Note that VLO+ and VLO− are complementary: when VLO+ is high, VLO− is low; and when VLO+ is low, VLO− is high. As a result, when VLO+ is high (and therefore VLO− is low), VI+ and VI− are coupled to VO+ and VO− via M1 and M3, respectively; when VLO+ is low (and therefore VLO− is high), VI+ and VI− are coupled to VO− and VO+ via M2 and M4, respectively. Consequently, the output differential signal (VO+-VO−) is approximately equivalent to the input differential signal (VI+-VI−) multiplied by 1 (when VLO+ is high) or −1 (when VLO+ is low). Therefore, the output either tracks the input, or tracks the inversion of the input, depending on the state of the LO signal. In a direct-down conversion application, the input differential signal represents the RF signal, and the output differential signal represents the baseband signal, which is approximately equal to the RF signal multiplied by the LO signal.
Many mixer circuits are constructed using the commutation network 100 of FIG. 1. Among them, a mixer is generally classified as “active” if there is a DC (direct current) flowing between the input side and the output side. On the other hand, a mixer is generally classified as “passive” if there is no DC flowing between the input side and the output side. FIG. 2 depicts a prior art passive mixer 200 based on using a commutation network 210. On one side, instead of directly coupling the commutation network 210 to the input differential signal comprised of VI+ and VI−, a pair of capacitors C3 and C4 are used to provide AC (alternate current) coupling. On the other side, a termination network (for examples: a pair of parallel RC networks, R1-C1 and R2-C2) is used to provide termination for the output differential signal comprised of VO+ and VO−. Here, VCM is a DC voltage provided to set the common-mode voltage for the output differential signal VO+ and VO−. Due to the AC coupling capacitors C3 and C4, there is no DC flowing between the input side and the output side. Since there is no DC flowing between the two sides of the commutation network 210, passive mixer 200 allows separate biasing conditions for both sides of networks. This offers an advantage in flexibility that is lacked in an active mixer, where both sides of networks cannot be biased separately due to the DC flowing between them.
There is a problem with the prior art passive mixer 200. Under a static bias condition, for each of the four MOS transistors (M1 to M4), both sides of its terminals are biased to the common-mode voltage VCM. A MOS transistor provides a channel for current to flow between a first terminal and a second terminal, where a third terminal (i.e. the “gate”) controls the conductivity of the channel. The channel, however, is symmetrical and there is no physical difference between the first terminal and a second terminal. The current can flow from the first terminal to the second terminal, and vice versa. Between the first terminal and the second terminal, one is defined as the “source” and the other is defined as the “drain,” depending on the direction of the current flow. Under a static bias condition, there is no current flowing in any of the four MOS transistors for the prior art passive mixer 200. Under a dynamic condition where an AC (alternating current) RF signal is applied to the input side, there is an AC flowing in each of the four MOS transistors whenever its respective controlling signal (VLO+ or VLO−) is high. Since the current flow is alternating in nature, the definition of source and drain is also alternating. When the current flow changes direction, the roles of the previously “source” and the previously “drain” are also reversed. This phenomenon is called drain-source reversal phenomenon, or reversal phenomenon for short. The conductivity of the channel is primarily determined by the voltage difference between the gate and the source. Whenever the reversal phenomenon takes place, there is usually an abrupt change in the conductivity of the channel. This leads to distortion to the output signal.
What is needed is a method to alleviate the adverse effect of the reversal phenomenon.